Photonic Crystal Geometry for Organic Solar Cells

Abstract

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (∼4 cm 2 ) that is scalable. We show efficiency improvements of ∼70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations.Incommensurate length scales conspire to degrade photovoltaic efficiencies in organic solar cells. Exciton diffusion lengths (∼10 nm) are an order of magnitude smaller than absorption lengths. 1,2 This discrepancy is ameliorated by coprecipitating a bicontinuous donor and acceptor phase, a disordered bulk heterojunction (BHJ), 1,3 but inherent disadvantages persist: low exciton dissociation probability, 4 mismatched band gaps, 5 and optical losses. 6 Optimized BHJ devices have active layer thicknesses of ∼100 nm due to the device-limiting trade-off between optical absorption and electrical performance. 7,8 These thicknesses are adequate to absorb most photons in the visible range of the solar spectrum due to the strong extinction coefficient of contemporary BHJ materials. 2,9 However, the sun's maximum photon flux is located around λ ) 700 nm, which is near the band edge of many BHJ materials where absorption is weak. Energy-level engineering via custom synthesis 5 has produced so-called low-bandgap polymers 10 that exhibit broader absorption tails but are located further toward the near-infrared. The active layer thickness constraints imposed on BHJ solar cells make it imperative to develop innovative ways to enhance absorption in a specific spectral range with weak absorption without increasing photoactive layer thickness.Light trapping schemes based on ray optics have provided enhancement in optical absorption, e.g. collector mirrors, 11 microprism substrates, 12 and V-folded configurations, 13 but in these schemes absorption enhancement was not tailored to a desired spectral range. Methods based on wave optics have shown greater promise, and both diffraction gratings [14][15][16] and photonic crystal (PC) designs have been investigated. In particular, theoretical considerations of PC geometries suggest efficiency improvements would be anticipated [17][18][19] for organic solar cells similar to theoretical work on inorganic devices. 20,21 In spite of this theoretical promise, there has been little progress in experimentally demonstrating PC effects in organic solar cells. While there has been success in nanopatterning the photoactive layer, the primary intention has been to provide an undulating surface for evaporating a ...

Topological Nature of Optical Bound States in the Continuum

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Fluorescent polystyrene photonic crystals self-assembled with water-soluble conjugated polyrotaxanes

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Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography

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